Etching process for micromachining materials and devices fabricated thereby

ABSTRACT

The present invention provides an optical microbench having intersecting structures etched into a substrate. In particular, microbenches in accordance with the present invention include structures having a planar surfaces formed along selected crystallographic planes of a single crystal substrate. Two of the structures provided are an etch-stop pit and an anisotropically etched feature disposed adjacent the etch-stop pit. At the point of intersection between the etch-stop pit and the anisotropically etched feature the orientation of the crystallographic planes is maintained. The present invention also provides a method for micromachining a substrate to form an optical microbench. The method comprises the steps of forming an etch-stop pit and forming an anisotropically etched feature adjacent the etch-stop pit. The method may also comprise coating the surfaces of the etch-stop pit with an etch-stop layer.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/199,476,filed Jul. 19, 2002, now U.S. Pat. No. 6,907,150, which is acontinuation-in-part of application Ser. No. 10/071,261 filed on Feb. 7,2002, now U.S. Pat. No. 6,885,786 issued Apr. 26, 2005 which in turnclaims the benefit of priority of U.S. Provisional Application No.60/266,931, filed on Feb. 7, 2001, the entire contents of which areincorporated herein by reference. Applicants claim the benefit ofpriority of U.S. Provisional Application No. 60/306,568, filed on Jul.19, 2001, the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to devices having intersectingstructures etched into a substrate and a method for making such devices,and more specifically to structures having a planar surface formed alonga selected crystallographic plane of a single crystal substrate, wherethe method provides that such a surface retains a selected planarorientation at the point of intersection of such a surface with surfacesof additional structures in the substrate so that, for example, perfectconvex corners are maintained.

BACKGROUND OF THE INVENTION

The ability to precisely locate optical elements relative to one anotheris of critical importance in the fabrication of micro-optical devices,since the alignment tolerances between elements are often specified insubmicron dimensions. Typically, such elements may include an opticalsignal source, such as a laser, a detector, and an integrated ordiscrete waveguide, such as a fiber-optic or GRIN rod lens.Additionally, such elements may include a fiber amplifier, opticalfilter, modulator, grating, ball lens, or other components for conveyingor modifying an optical beam. Micro-optical devices containing suchcomponents are crucial in existing applications such as opticalcommunication and consumer opto-electronics, as well as applicationscurrently being developed, such as optical computing.

Maintaining precise alignment among the optical elements may beconveniently provided by an optical microbench, such as a siliconoptical bench. An optical microbench comprises three-dimensionalstructures having precisely defined surfaces onto which optical elementsmay be precisely positioned. One material well-suited for use as anoptical microbench is single crystal silicon, because single crystalsilicon may be etched anisotropically to yield three-dimensionalstructures having planar sidewalls formed by the precisely definedcrystallographic planes of the silicon. For example, the {111} siliconplane is known to etch more slowly than the {100} or {110} planes withproper choice of etchant. Thus, structures may be formed comprisingwalls that are {111} planes by anisotropic etching.

Since the optical elements sit within the three-dimensional structuresat a position below a top surface of the silicon substrate, a portion ofthe optical path often lies below the top surface of the substrate,within the volume of the substrate. Accordingly, passageways must beprovided in the optical microbench between three-dimensional structuresso that light may travel between the elements disposed in the associatedthree-dimensional structures. Hence, an optical microbench shouldcontain three-dimensional structures that communicate with one anotherthrough structures such as a passageway.

While discrete, non-communicating, three-dimensional structures may beconveniently formed by an isotropic etching, etched structures whichcommunicate with one another at particular geometries, such as a convexcorner, pose significant problems for applications in which it isdesirable to maintain the precise geometry defined by thecrystallographic planes. For example, where two {111} planes intersectat a convex corner, the convex corner does not take the form of astraight line intersection between two planes, but rather rounds tocreate a rounded intersection between the two {111} planes. As etchingcontinues to reach desired depth of the structure containing the {111}planes, the rounding of the corners can grow to such an extent that asubstantial portion of the intersection between the two {111} planes isobliterated. Since the {111} planes are provided in thethree-dimensional structures to form a planar surfaces against whichoptical elements may be precisely positioned, absence of a substantialportion of the {111} planes at the intersection can introduce a greatdeal of variability of the positioning of the elements at theintersection. Thus, the benefits provided by the crystallographic planescan be unacceptably diminished.

Traditionally, to avoid etching intersecting features, dicing saw cutsmay be used. However, dicing saw cuts can be undesirable, because suchcuts typically must extend across the entire substrate and may notconveniently be located at discrete locations within the substrate.Moreover, dicing saw cuts create debris which may be deposited acrossthe substrate surface and lodge within the three-dimensional structures,which may interfere with the precise positioning of optical elementswithin such a structure.

Therefore, there remains a need in the art for optical microbenchtechnology which permits three-dimensional structures havingcrystallographic planar surfaces to intersect with other surfaces,without degrading the crystallographic orientation of the intersectedplanar surfaces.

SUMMARY

The present invention provides an optical microbench comprising asubstrate having an etch-stop pit and an etched feature, such as ananisotropically etched feature, disposed adjacent the etch-stop pit. Theanisotropically etched feature may comprise a V-groove. The etch-stoppit may have a shape suited to supporting an etch-stop layer on thesurfaces of the etch-stop pit. The etch-stop pit may be created prior tocreating the etched feature. The etch-stop layer comprises a materialresistant to the etchant which is used to create the etched feature.After the etch-stop layer is provided, the etched feature is etched inthe substrate. The etch-stop layer prevents the feature etching fromextending into the region where the etch-stop pit is located. Theprevention of such etching by the etch-stop layer provides that thecrystallographic planner walls of the anisotropically etched featuremaintain their crystallographic orientation adjacent the stop-etch pit.

The present invention also provides a method for micromachining asubstrate to form an optical microbench. The method comprises the stepsof forming an etch-stop pit and forming an anisotropically etchedfeature adjacent the etch-stop pit. The method may also comprise coatingthe surfaces of the etch-stop pit with an etch-stop layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following detailed description of thepreferred embodiments of the present invention will be best understoodwhen read in conjunction with the appended drawings, in which:

FIG. 1 schematically illustrates a top view of a V-groove provided in anupper surface of a substrate, where the V-groove includes two ends whichinclude wedge-shaped end portions;

FIGS. 2A–2D schematically illustrate top views of a substrate showingthe changes to the substrate as features are added to the substrate tocreate a structure having a V-groove and an adjoining etch-stop pit inaccordance with a first embodiment of the present invention;

FIGS. 3A–3F and 4A–4D schematically illustrate top views of alternativeetch-stop pit configurations with the adjoining V-grooves in accordancewith the present invention;

FIG. 5 schematically illustrates a top view of an alternative etch-stoppit configuration in accordance with the present invention forpreventing formation of a wedge-shaped end wall in a V-groove;

FIG. 6 schematically illustrates a top view of an alternative etch-stoppit configuration in accordance with the present invention for providinga partial, wedge-shaped end wall in a V-groove;

FIG. 7 schematically illustrates a top view of an alternative etch-stoppit configuration in accordance with the present invention forpreventing formation of a wedge-shaped end walls in three V-grooveswhich adjoin the etch-stop pit;

FIGS. 8–10 schematically illustrate top views of further configurationsof etch-stop pits and V-grooves along with a device mount for providingoptical subassemblies in accordance with the present invention;

FIGS. 11–16 schematically illustrate top views of alternativeconfigurations etch-stop pits with two or more V-grooves adjoining theetch-stop pits;

FIGS. 17–20 schematically illustrate top views of substrates havingetch-stop pits, V-grooves, and an optional V-pit, for providing opticalsubassemblies in accordance with the present invention;

FIGS. 21–26 schematically illustrate top views of substrates having twoV-grooves oriented at 90 degrees with respect to one another and havingan etch-stop pit disposed at the location of the intersection of the twoV-grooves;

FIGS. 27 and 28 schematically illustrate top views of substrates havingan etch-stop pits disposed at locations where an inside, convex cornerof two intersecting V-grooves would be located;

FIGS. 29A–29D, 30, 31, 32A–32B, and 33 schematically illustrate topviews of substrates having an etch-stop pit which circumscribes aselected area of the substrate in which an anisotropically etchedfeature is formed;

FIGS. 34, 35A, 36, and 37 schematically illustrate top views ofsubstrates having a U-shaped etch-stop pit adjacent to a V-pit toprovide a location on the substrate for a laser mount and to provide alocation for retaining a spherical optical element;

FIG. 35B schematically illustrates a cross-sectional view of thesubstrate illustrated in FIG. 35A;

FIGS. 38A and 39A schematically illustrate top views of substrateshaving a V-groove with an etch-stop pit and fiber stop disposedinternally to the groove;

FIGS. 38B and 39B schematically illustrate cross-sectional views of thesubstrates illustrated in FIGS. 38A and 39A, respectively;

FIG. 40 illustrates a flowchart representing a process in accordancewith the present invention for creating an etch-stop pit and an adjacentanisotropically etched feature;

FIG. 41 illustrates a flowchart representing another process of thepresent invention for creating an etch-stop pit and adjacent ananisotropically etched feature;

FIG. 42 illustrates a flowchart representing yet another process of thepresent invention for creating an etch-stop pit and an adjacentanisotropic feature;

FIGS. 43 and 44 schematically illustrate a top view and across-sectional view, respectively, of a substrate comprising a V-grooveand an adjoining etch-stop pit;

FIGS. 45–51 schematically illustrates cross-sectional views of asubstrate at selected steps of processing in accordance with the methodillustrated in the flowchart of FIG. 41;

FIGS. 52–58 schematically illustrates cross-sectional views of asubstrate at selected steps of processing in accordance with the methodillustrated in the flowchart of FIG. 42; and

FIGS. 59–64 schematically illustrates cross-sectional views of asubstrate at selected steps of processing in accordance with the methodillustrated in the flowchart of FIG. 43.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, wherein like elements are numbered alikethroughout, several different embodiments of devices in accordance withthe present invention are illustrated. The different embodiments includea substrate having at least two common features, an etch-stop pit and ananisotropically etched feature adjacent the etch-stop pit. The etch-stoppit has a shape suited to supporting an etch-stop layer on the surfacesof the etch-stop pit. The etch-stop pit is created prior to creating theanisotropically etched feature. The etch-stop layer comprises a materialresistant to the etchant which is used to create the anisotropicallyetched feature. After the etch-stop layer is provided, theanisotropically etched feature is etched in the substrate. The etch-stoplayer prevents the anisotropic etching from extending into the regionwhere the etch-stop pit is located. The prevention of such anisotropicetching by the etch-stop layer provides that the crystallographicplanner walls of the anisotropically etched feature maintain theircrystallographic orientation adjacent the stop-etch pit. The advantagesof preventing such etching are illustrated in the accompanying figuresdepicting several desirable embodiments of the present invention.

Throughout the figures, the substrate material is selected to be<100>-oriented silicon. However, other orientations of silicon, such as<110>-oriented silicon, may also be used in accordance with the presentinvention. In addition, other anisotropic crystalline materials, such asIII–V semiconductor materials, e.g., InP, GaAs, InAs, or GaP, may beused in accordance with the present invention. The substrate material ischosen with regard to the nature of the particular optical device andthe features to be fabricated. The crystal orientation of the substratemay be chosen with respect to the desired orientation of the sidewallsof the fabricated features. For example, <100>-oriented silicon may beselected to create a features having sidewalls that are sloped withrespect to the upper surface of the substrate. Alternatively,<110>-oriented silicon may be selected to create features havingsidewalls that are perpendicular to the upper surface of the substrate.

An example of a typical feature which may be formed by anisotropicetching in an <100>-oriented silicon substrate 6 is a V-groove 2, asillustrated in FIG. 1. In a first aspect of the present invention amodified V-groove 2 is provided, V-groove 12, having a configurationparticularly well-suited to retaining a cylindrical element, such as anoptical fiber or GRIN rod lens, as illustrated in FIGS. 2A–2D.

Turning first to the V-groove 2 illustrated in FIG. 1, each surface ofthe V-groove 2 is a {111}-plane of the silicon substrate 6. The V-groove2 may be made by known methods such as etching through a rectangularaperture mask using an aqueous solution of KOH. A V-groove 2 which doesnot extend to the edges of the substrate 6 includes two wedge-shaped endwalls 4. The end walls 4 slope upwardly towards the upper surface 1 ofthe substrate 6 from an apex 5. A wedge-shaped end wall 4 is oftenundesirable in optical subassemblies, because a wedge-shaped end wall 4can partially or completely occlude the optical path to block lighttransmitted to or from an optical element disposed in the V-groove 2. Inaddition, the wedge-shaped end wall 4 functions poorly as an opticalfiber stop, since the wedge-shaped end wall 4 is sloped with respect tothe endface of the fiber which is usually perpendicular to thelongitudinal axis of the fiber. Thus, it is desirable to create aV-groove without one or more of the wedge-shaped end walls 4.

In particular, referring to FIG. 2D, a V-groove structure in accordancewith the present invention is illustrated where one of the wedge-shapedend walls 14 a, shown in phantom, is eliminated from the V-groove 12.The device includes a substrate 10 having an upper surface 11 in which aV-groove 12 and adjacent etch-stop pit 16 are provided. The edges 13where the V-groove 12 and the stop-etch pit 16 intersect are straightline segments that lie within the {111}-plane of the V-groove sidewalls15. The ability to remove the right end wall 14 a while maintaining the{111}-orientation of the sidewalls 15 in the vicinity of the removed endwall 14 a is provided by the etch-stop pit 16 and etch-stop layer 18.

The sequence in which the etch-stop pit 16 and V-groove 12 are formed inthe surface of the substrate 10 is illustrated in FIGS. 2A–2D. Turningto FIG. 2A, a top elevational view of the substrate 10 is shown in whichan etch-stop pit 16 is formed. As depicted, the etch-stop pit 16 has atriangular cross-section in the plane of the upper surface 11 of thesubstrate 10. Other shapes than triangular cross-section may be used solong as such shapes are suited to preventing the formation of awedge-shaped end wall 14 a of the V-groove 12. The types of shapes whichmay be used are discussed below with reference to FIGS. 3 and 4.

The walls of the etch-stop pit 16 may desirably extend into thesubstrate 10 at a 90 degree angle, i.e. vertical, relative to the uppersurface 11 of the substrate 10, and the etch-stop pit 16 may contain aflat bottom parallel to the plane of the upper surface 11. Such aconfiguration of the etch-stop pit 16 may be fabricated by high-aspectratio dry etching, such as reactive ion etching. Alternatively, theetch-stop pit 16 may include sidewalls that are sloped with respect tothe plane of the upper surface 11. Regardless of the sidewall slope thatis utilized, the portions of the sidewalls 16 a located proximate theregion at which the V-groove 12 is to be formed, i.e. at intersectingsegments 13, should extend into the substrate 10 a greater depth thanthe depth intended for the adjoining portion of the V-groove 12.Providing such a deeper sidewall portion ensures that a subsequentlyapplied etch-stop layer 18 provides a barrier between an etchant in theV-groove 12 and the etch-stop pit 16.

After formation of the etch-stop pit 16, an etch-stop layer 18 isconformally provided on the sidewalls 16 a and the bottom of theetch-stop pit 16. The etch-stop layer 18 comprises a material that isresistant to the etchant that will be used to create the V-groove 12.For example, the etch-stop layer 18 may comprise silicon dioxide, whichmay be provided by CVD or by thermally oxidizing surfaces 16 a of theetch-stop pit 16, or silicon nitride, which may be provided by CVD.Optionally, the upper surface 11 of the substrate 10 may be providedwith a layer of the same material used for the etch-stop layer 18.

The V-groove 12 is formed in the surface 11 of the substrate 10 by asuitable process, such as anisotropic wet etching with KOH or EDPthrough a rectangular aperture mask. The rectangular aperture mask isoriented such that the perimeter of the rectangular aperture isregistered to the perimeter of the V-groove 12 located in the uppersurface 11 of the substrate 10. The rectangular aperture mask isoriented such that a portion of an end of the rectangular apertureoverlies the etch-stop pit 16. Further details regarding how the masksare provided is discussed below in connection with the method of thepresent invention.

As an optional additional step, the etch-stop layer 18 may be removedfrom the etch-stop pit 16. Removal of the etch-stop layer allows theV-groove 12 to communicate with the etch-stop pit 16. Such communicationpermits an element, such as a fiber, disposed within the V-groove 12 toextend into the region over the etch-stop pit 16 and abut the sidewall16 a of the etch-stop pit 16 that is disposed perpendicular to thelongitudinal axis of the V-groove 12, to provide a fiber stop 17.

The process described above with respect to FIG. 2D is suited to formingall of the structures illustrated herein. For example, each of thefollowing structures described below includes at least one etch-stop pitwhich is formed before an adjacent anisotropically etched feature, suchas a V-groove is formed adjacent to the pit. In addition, an etch-stoplayer is provided in the etch-stop pit prior to forming aanisotropically etched feature. While the etch-stop layer may not beillustrated in the figures, because it has been removed after theformation of the anisotropically etched feature, it is understood thatthe etch-stop layer is present within the etch-stop pit while theanisotropically etched feature is being formed.

In addition to the triangular cross-sectional shape of the etch-stop pit16 illustrated in FIGS. 2A–2D, other cross-sectional shapes may be usedin accordance with the method of the present invention to completely orpartially prevent the formation of a wedge-shaped end wall of aV-groove, as shown in FIGS. 3C–3F and 4A–4D. For example, a first typeof etch-stop pit configuration for completely preventing the formationof a wedge-shaped end wall 44 is illustrated in FIGS. 4A–4D. FIGS. 4A–4Dillustrate top elevational views of a V-groove 42 adjacent to etch-stoppits 46, 47, 48, 49 of differing cross-sectional areas. In eachconfiguration, the etch-stop pit 46, 47, 48, 49 completely overlays aregion of the substrate in which the wedge-shaped end wall 44 of theV-groove 42 would otherwise be formed. The etch-stop pit 46, 47, 48, 49and V-groove 42 may be formed in the substrate by the proceduredescribed above with respect to the device illustrated in FIG. 2D.

One desirable configuration of an etch-stop pit 49 comprises twosidewalls joined at an apex that lies along the longitudinal axis of theV-groove 42 such that the apex angle, α, is bisected by the longitudinalaxis. Such a configuration of an etch-stop pit 49 can prevent theformation of a wedge-shaped end wall 44 when the apex angle is less thanor equal to 90 degrees.

If the apex angle were greater than 90 degrees, as illustrated in FIGS.3C and 3D, a partial wedge-shaped end wall 34 would be formed in theV-groove 32. In the configuration where the “apex angle” is equal to 180degrees, i.e. a straight line, the typical wedge-shaped end wall 24would be formed in the V-groove 22, as illustrated in FIGS. 3A and 3B.That is, an etch-stop pit 26 having a straight sidewall 23 adjacent tothe area in which the V-groove 22, is to be formed, and orientedperpendicular to the longitudinal axis of the V-groove 22, allows forthe formation of a wedge-shaped end wall 24. Other cross-sectionalshapes of an etch-stop pit are contemplated in accordance with thepresent invention, such as the “W” cross-sectional shape depicted inFIGS. 3E and 3F.

Yet another configuration of an etch-stop pit 386 in accordance with thepresent invention may be provided so that a fiber stop 387 is createdwithin a V-groove 384, as illustrated in FIGS. 38A–38B and 39A–39B.FIGS. 38A and 39A illustrated top views of a substrate 380 in which aV-groove 384 is formed. FIGS. 38B and 39B illustrate cross-sectionalviews taken along the lines B—B in FIGS. 38A and 39A, respectively. Theetch-stop pit 386 has a shape that promotes the formation of awedge-shaped fiber stop 387 along a {111} crystallographic planeadjacent a first sidewall 383 of the etch-stop pit 386. In particular,the straight sidewall 383 oriented perpendicular to the longitudinalaxis of the V-groove 384 promotes the formation of the wedge-shapedfiber stop 387 in an analogous fashion to the formation of thewedge-shaped end wall 24 in FIG. 3B. The etch-stop pit 386 alsocomprises a pair of angled sidewalls 385 across the dark and 386 fromthe first end wall 383. The angled sidewalls 385 intersect at a selectedapex angle which has a magnitude and orientation suitable for preventingthe formation of wedge-shaped surfaces, i.e. {111} surfaces, in theV-groove 384 adjacent to the angled sidewalls 385. The angled sidewalls385 may have a similar configuration to corresponding sidewalls depictedin FIG. 2D. As illustrated in the cross-sectional views of FIGS. 38B and39B, the wedge-shaped fiber stop 387 extends above the deepest portionof the V-groove 384 so that a fiber 381 disposed within the V-groove 384may abut the wedge-shaped fiber stop 387.

A second type of etch-stop pit configuration that prevents awedge-shaped end wall from forming has a parallelogram cross-sectionalshape oriented at an angle, β, of 45 degrees or less, with thelongitudinal axis of the V-groove 52, as illustrated in FIG. 5. If, theangle, β, is larger than 45 degrees, as depicted in the configuration ofFIG. 6, then a partial wedge-shaped end wall 64 is formed in theV-groove 62. In a case where β is 90 degrees, the configuration of theetch-stop pit becomes functionally equivalent to that of the etch-stoppit depicted in FIG. 3A. In addition, V-grooves 52, 53, 55 may beprovided on opposing sides of the etch-stop pit 56 as illustrated inFIG. 7. So long as the longitudinal axis of each V-grooves 52, 53, 55 isoriented at an angle less than 45 degrees relative to an adjacentsurface of the etch-stop pit 56, the etching process in accordance withthe present invention will not produce wedge-shaped end walls in theV-grooves 52, 53, 55 in the region adjacent the etch-stop pit 56. Anynumber of V-grooves may be so provided, and such grooves need not havethe same size.

Returning now to the configuration illustrated in FIG. 2D, where thecombined V-groove 14 and etch-stop pit 16 provide a cavity having afiber stop 17 for retaining a fiber optic, further optical subassembliesmay be fabricated by providing additional features in or on thesubstrate 10. Such subassemblies may provide for optical communicationwith the fiber. In particular, the structure of FIG. 2D is well-suitedfor use with other optical elements, because the fiber stop 17 providesa fiducial reference point to precisely identify where the end of thefiber is located.

For example, FIGS. 8–10 illustrate top elevational views of additionalconfigurations in accordance with the present invention that provideoptical subassemblies comprising a fiber 81, 91, 101, a V-groove 84, 94104, and a laser mount 85, 95, 105. Alternatively, detector mounts couldbe provided in place of the laser mounts 85, 95, 105. In particular,with reference to FIG. 8, a V-groove 84 and adjoining etch-stop pit 86with fiber stop 83 are provided in a configuration similar to thatdepicted in FIG. 2D described above. The etch-stop pit 86, however, isnot precisely triangular in cross-section, but rather includes an etchedarea 87 that protrudes, in cross-section, from the fiber-stop edge ofthe etch-stop pit 86, so that the cross-sectional shape of the etch-stoppit 86 is similar to that of an arrowhead. The etched area 87 allows forbeam expansion. In addition, a laser mount 85 is provided proximate theetched area 87 and is disposed along the longitudinal axis of theV-groove 84. It may be desirable to provide an optical device betweenthe end of the fiber optic 81 and the laser mount 85. Accordingly, theconfigurations illustrated in FIGS. 9 and 10 provide slots 99, 109 forreceiving optical elements. The slots 99, 109 communicate with therespective etch-stop pits 96, 106 and may be formed at the same time asthe etch-stop pits 96, 106. The slots 99, 109 comprise verticalsidewalls, however, sloped sidewalls may also be provided. The slot 109of FIG. 10 conveniently has a cross-sectional shape of a plano-convexlens, whereas the slots 99 is well-suited to receiving flat optics.

In yet another etch-stop pit configuration in accordance with thepresent invention, the etch-stop pit may have a diamond-likecross-sectional shape which is suited to device configurations thatinclude two or more V-grooves disposed on opposing sides of theetch-stop pit, as illustrated in FIGS. 11–16. Referring to FIG. 11, asubstrate 110 is shown which includes an a diamond cross-sectionalshaped etch-stop pit 116 with two V-grooves 114, 115 disposed onopposing sides of the etch-stop pit 116. The V-grooves 114, 115 havelongitudinal axes are collinear and intercept at a respective vertex ofthe etch-stop pit 116. The region of intersection between each V-groove114, 115 with the respective portion of the etch-stop pit 116, has asimilar geometry to the intersection between the V-groove 14 andetch-stop pit 16 depicted in FIG. 2D. Thus, for the same reasons givenabove, no wedge-shaped end wall is formed in the V-grooves 114, 115 atthe locations adjacent the etch-stop pit 116. To allow the end faces ofrespective fibers disposed in two V-grooves 164, 165 to be space moreclosely together, the etch-stop pit 166 may comprise a diamond-likeshape that is compressed, as illustrated in FIG. 16.

In a similar manner, the etch-stop pit 136 may have a cross-sectionalshape suited to having a single V-groove 134 on one side of theetch-stop pit 136 and having two or more V-grooves 135, 137, 139,disposed at an opposing side of the etch-stop pit 136. In addition, theetch-stop pit 136 may have a cross-sectional shape suited to preventingthe formation of a wedge-shaped end wall in each V-groove 134, 135, 137,139 at the respective positions where the V-grooves 134, 135, 137, 139adjoin the etch-stop pit 136. A suitable shape for such an etch-stop pit136 as depicted in FIG. 13. The etch-stop pit 136 provides two fiberstops 137 for a fiber disposed in the V-groove 134. Yet additionalshapes of an etch-stop pit 126 may be provided for preventing theformation of wedge-shaped end walls in multiple V-grooves 124, 125, 126,127, as illustrated in FIG. 12. Wedge-shaped end walls do not form forthe reasons given above with regard to FIGS. 4D and 7, for example.

Still further, two of the ‘etch-stop pit with adjoiningV-groove’-structures illustrated in FIG. 2D may be provided in a singlesubstrate 140 in back-to-back coaxial relationship with a passageway 149extending between the two triangular sections of the etch-stop pit 146,as illustrated in FIG. 14. Such a configuration provides a fiber stop147 for each of the V-grooves 144, 145 so that the distance, D, betweenthe ends of two fibers located within the V-grooves 144,145 may beprecisely specified. In addition, the passageway 159 may be sufficientlylong so as to provide for insertion of an optical element between thetwo fiber stops 157. A slot 153, or other suitable shape, is provided toreceive such an optical element.

In yet another aspect of the present invention, two or more theabove-described ‘etch-stop pit with adjoining V-groove’ structures maybe provided in a substrate with a V-pit disposed therebetween, as shownin FIGS. 17, 18, and 20. A V-pit 179, 189, 209 may be formed byanisotropic etching by the same methods used to form V-grooves but usinga square aperture mask rather than a rectangular aperture mask. TheV-pit 179, 189, 209 may be anisotropically etched at the same time asthe grooves 174, 184, 204. The V-pit 179, 189, 209 should be etchedafter the etch-stop pit 176, 186, 206 and the etch-stop layer areprovided, in accordance with the process described above in reference toFIG. 2D. The V-pit 179, 189, 209 comprises for triangular-shaped,sidewalls that lie in the {111} crystallographic planes to form afour-sided regular pyramid that extends into the substrate 170, 180,200. Like the V-grooves 174,184 the V-pits 179,189 should extend intothe substrate a depth less than the depth of the etch-stop pits 176, 186at the point of intersection between the V-pits 179, 189 and theetch-stop pits 176, 186, as illustrated in FIGS. 17 and 18. In aconfiguration where the V-pit 209 does not intercept the etch-stop pit206, the V-pit 209 depth does not need to be selected with regard to thedepth of the etch-stop pit 206. The V-pits 179, 189, 209 provide aconvenient shape for retaining a spherical optical element the V-pits179, 189, 209, such as a ball lens. The V-grooves 174, 184, 204 arepositioned so that an optical element disposed within the V-grooves 174,184, 204 can optically communicate with the optical element disposedwithin the respective V-pit 176, 186, 206. In alternative configuration,as illustrated in FIG. 19, the etch-stop pit 196 may contain a centralportion 195 configured to hold a spherical optical element. For example,the central portion may have a diamond-like shape. The central portion195 of the etch-stop pit 196 may serve the same function of retaining aspherical optical element as that of the V-pit 189.

In another aspect of the present invention, an etch-stop pit may beprovided at a location where two anisotropically etched features wouldintersect to form an inside, convex corner. A convex corner formed bythe intersection of two {111} planes does not form a straight lineintersection between two planes, but rather creates a roundedintersection between the two intersecting {111} planes. The rounding canpropagate to remove material in the vicinity of the intersection, suchthat the well-defined {111} planes can be etched away in the vicinity ofthe intersection to yield structures that are not {111} planes. Thus, itwould be desirable to prevent the formation of such rounded corners.

FIGS. 21–28 illustrate several configurations of etch-stop pits inaccordance with the present invention which are suited to preventundesirable etching at an inside, convex corner. Each of the structuresin FIGS. 21–28 may desirably be formed by the process described abovewith reference to FIG. 2D, with an etch-stop pit and etch-stop layerprovided in the substrate prior to anisotropically etching theV-grooves. For example, referring to FIG. 21, a top elevational view ofthe substrate 210 is shown in which two V-grooves 214 are disposed. Thetwo V-grooves 214 are oriented with their respective longitudinal axesat 90 degrees relative to one another. An etch-stop pit 216 is providedat a selected location of the substrate 210 corresponding to thelocation at which the two V-grooves 214 would otherwise intersect.Providing the etch-stop pit 216 at the selected location preventsintersection of the V-grooves 214. The etch-stop pit 216 may be disposedat a 45 degree angle, β, so that wedge-shaped end walls are not formedin the V-grooves 214 adjacent the etch-stop 216. To provide for greaterease of alignment (lower alignment tolerances) between the etch-stop pit216 and the longitudinal axis of the V-grooves 214, an angle of lessthan 45 degrees may be preferable.

Alternative configurations of an etch-stop pit that prevent etching ofan inside, convex corner and wedge-shaped V-groove end walls areillustrated in FIGS. 22–28. Each configuration depicted in FIGS. 22–28includes V-grooves 224, 234, 244, 254, 264 oriented at 90 degrees withan intermediate etch-stop pit 226, 236, 246, 256, 266 in a similarconfiguration to that of FIG. 21. The etch-stop pit 226, 236, 246, 256,266 is located to prevent intersection of the V-grooves 224, 234, 244,254, 264. Each of the etch-stop pits 226, 236, 246, 256, 266 hasstraight wall segments disposed at an angle of 45 degrees or less withrespect to the longitudinal axis of an adjacent V-groove 224, 234, 244,254, 264. Referring to FIGS. 22 and 23, the etch-stop pit 226, 236 mayinclude an interior portion 225, 235 for retaining optical element suchas filters, lenses, micromechanical switches, for example. In addition,as illustrated in FIGS. 25 and 26, the etch-stop pit 256, 266 may have ashape configured to provide a fiber stop 257, 267 for fibers 251, 261disposed within the V-grooves 254, 264.

In accordance with the present invention, yet additional configurationsof etch-stop pits 276, 286 are provided which permit the intersection oftwo V-grooves 274, 284 while preventing the formation of an inside,convex corner 275, 285, thus obviating the need for corner compensation,as illustrated in FIGS. 27 and 28. For example, a top elevational viewof a substrate 270, 280 is shown in which pairs of V-grooves 274, 284are provided in an upper surface of the substrate 270, 280. Pairs ofV-grooves 274, 284 intersect at ends of the V-grooves 274, 284 at anangle of 90 degrees. An etch-stop pit 276, 286 is provided at a selectedlocation of the substrate 270, 280 corresponding to the location atwhich an inside corner 275, 285 of the intersecting V-grooves 274, 284would otherwise be formed. Providing the etch-stop pit 276, 286 coatedwith an etch-stop layer at the selected location prevents formation ofthe inside convex corner 275, 285.

In a further aspect of the present invention, an etch-stop pit 296 maybe provided as a continuous boundary that circumscribes a region of thesubstrate 294 that is to be anisotropically etched. Providing such anetch-stop pit boundary permits the anisotropically etched features to beetched more deeply than otherwise possible. For example, referring toFIG. 30, a cross-sectional view of a substrate 300 is shown in which arecessed V-groove 304 is provided. The ability to form the V-groove 304below the plane of the upper surface 301 is provided by the presence ofthe etch-stop pit 306 (coated with an etch-stop layer) whichcircumscribes the region in which the V-groove 304 is formed. If theetch-stop pit 306 were not provided, the surfaces of the V-groove 304would extend upward to the upper surface 301 as indicated by the dashedline 307, and thus would not be recessed with respect to the uppersurface 301.

Turning now to FIGS. 29A–D, an L-shaped etch-stop pit 296 is providedwhich circumscribes an L-shaped area in which an anisotropically etchedfeature may be formed. Providing the L-shaped etch-stop pit 296 permitsthe formation of {111} sidewalls as illustrated in FIGS. 29A and 29B.Etching for a longer period of time permits a deeper feature to beformed, as illustrated in FIGS. 29C and 29D. Alternatively, other shapesthan L-shaped may be utilized as an etch-stop pit. For example, theetch-stop pit 316 may have a T-shaped cross-section as illustrated inthe top view of FIG. 31. Upon anisotropically etching the region 311bounded by the T-shaped etch-stop pit 316, {111} sidewalls may be formedas illustrated in FIGS. 32A and 32 B. In the vicinity of thecross-sectioning plane B—B, the anisotropically etched feature 324 mayhave a V-shaped cross-section, as illustrated in FIG. 32B. Yet furthershapes may be utilized in accordance with the present invention as anetch-stop pit which circumscribes an area to be anisotropically etched,such as the configuration depicted in FIG. 33.

In yet another aspect of the present invention, a U-shaped etch-stop pit346 is provided adjacent to a V-pit 345 to provide a location on asubstrate 340 for mounting an optical element, such as a laser mount355, and to provide a location for retaining an optical element, such asa spherical optical element 350, as illustrated in FIGS. 34–36. FIGS. 34and 35A illustrate a top view of a substrate 340 in which a U-shapedetch-stop pit 346 is provided adjacent a V-pit 345. The U-shapedetch-stop pit 346 includes sidewalls that extend a selected depth intothe substrate 340. Optionally, the sidewalls of the U-shaped etch-stoppit 346 may be vertical, as illustrated in FIG. 34. Alternatively, thesidewalls of the U-shaped etch-stop pit 346 may be inclined relative tothe upper surface 301 of the substrate 340. The sidewalls of theU-shaped etch-stop pit 346 are conformally coated with an etch-stoplayer, or, optionally, the etch-stop pit 346 is filled etch-stop layermaterial. In addition, the portion 343 of the substrate surface 301interior to the etch-stop pit 346 may be provided with an etch-stoplayer. As explained above with reference to the process of FIGS. 2A–2D,the etch-stop layer comprises a material that is resistant to theetching used to form an anisotropically etched feature, such as V-pit345.

After the desired etch-stop layer or layers are provided, the V-pit 345may be formed by anisotropic etching by the same methods used to formV-grooves but using a square aperture mask. Instead of using a perfectlysquare aperture mask, a generally square-aperture that includes aprotrusion to protect substrate surface portions 343 interior to theetch-stop pit 346 may be used. The V-pit 345 may be anisotropicallyetched at the same time as the optional V-groove 354. The V-pit 345should extend into the substrate a depth less than the depth of theetch-stop pit 346 at the point of intersection 353 between the V-pit 345and the etch-stop pit 346, as illustrated in FIG. 35B. In aconfiguration where the V-pit 345 does not intercept the etch-stop pit346, the V-pit 345 depth does not need to be selected with regard to thedepth of the etch-stop pit 346.

The V-pit 345 provides a convenient shape for retaining a sphericaloptical element, such as a ball lens 350. The interior portion 343 ofthe substrate surface 301 provides a convenient location at which alaser 355, or other optical device, may be located for opticalcommunication with the ball lens 350. Providing the U-shaped etch-stoppit 346 permits a portion of the V-pit 345 adjacent the etch-stop the346 to be recessed below the surface 341 of the substrate 340. Such arecess permits the ball lens 350 to be positioned more closely to thelaser 355, as illustrated in FIG. 35B. In addition, a V-groove 354, 374may also be provided for optical communication between a fiber disposedwith the V-groove 354, 374 and the V-pit, as illustrated in FIGS. 36 and37. With respect to FIG. 36, the V-groove 354 may be fabricated in asimilar manner as the V-grooves 174 of FIG. 17, for example.Alternatively, as illustrated in FIG. 37, the etch-stop pit 376 maycircumscribe the region in which the V-pit 375 is formed. The etch-stoppit 376 comprises a U-shaped segment 366 to provide an analogousfunction to that of the U-shaped etched pit 346 in the configuration ofFIG. 35A. The etch-stop pit 376 also comprises a triangular-shapedsegment 378 to prevent formation of a wedge-shaped end wall in theV-groove 374 and to provide a fiber stop 377.

Methods of Fabrication

In accordance with the present invention, there are provided methods forfabricating optical subassemblies having an etch-stop pit and anadjacent recessed area, such as an anisotropically etched area, forreceiving an optical element. Three exemplary methods are illustrated inthe flowcharts of FIGS. 40–42 and the accompanying side cross-sectionalviews of FIGS. 45–64. The orientation of the side cross-sectional viewsof FIGS. 45–64 is illustrated in FIGS. 43 and 44.

Referring to FIG. 43, a top elevational view is shown of a substrate 440in which a V-groove 444 and adjacent etch-stop pit 446 are provided. Thestructure shown in FIG. 43 is similar to that shown in FIG. 2D, whereone of the wedge-shaped end walls is eliminated from the V-groove 444. Across-sectional view taken along the line B—B is illustrated in FIG. 44to show a cross-section of the V-groove 444 at a location where theV-groove 444 intersects the etch-stop pit 446. FIGS. 45–64 illustratecross-sectional views of substrates which are taken along the same viewdirection, B—B, as the cross-sectional view in FIG. 44. The exemplarypart fabricated by each of the methods illustrated in the flowcharts ofFIGS. 40–42 has a final configuration similar to that of the deviceshown in FIGS. 43 and 44.

Referring now to FIG. 40, there is shown a flowchart illustrating amethod in accordance with the present invention for creating the deviceillustrated in FIGS. 43 and 44. As illustrated in FIG. 45, a substrate450 made from <100>-oriented Si is provided. The processing of thesubstrate 450 begins at step 4000 of FIG. 40 by providing a protectivelayer 452 on a first surface of the substrate 450 to cover that portionof the substrate 450 in which the etch-stop pit 516 is not to beprovided. That is, the protective layer 452 includes an etch-stop pitaperture 451 through which a portion of the substrate 450 surface isaccessible for forming the etch-stop pit 516.

The protective layer 452 may be deposited over the entire surface of thesubstrate 450. Thereafter, portions of the protective layer 452 may beremoved to expose the surface of the substrate 450 at the selected areafor the etch-stop pit 516. The material of the protective layer 452 ischosen to be resistant to the etchant that will be used to form theV-groove 512. For example, silicon dioxide is one suitable material. Thesilicon dioxide may be deposited by CVD or may be provided by thermaloxidation of the substrate surface. The silicon dioxide layer should bethick enough to serve as a mask during the etch-stop pit formation.

Following the application of the protective layer 452, an aperturedefinition layer 454 is deposited, at step 4010, over a selected portionof the protective layer 452, as shown in FIG. 45. The aperturedefinition layer 454 is provided so that an aperture 457 may beprovided, as explained below, through which the V-groove 512 will beetched. The location of the aperture definition layer 454 is selected tocover that portion of the substrate surface at which the V-groove 512 isto be located.

Processing continues with the selective removal, at step 4020, of aportion of the substrate 450 located within the etch-stop pit aperture451 to form an etch-stop pit 516 in the substrate 450, as depicted inFIG. 46. The etch-stop pit 516 may conveniently be formed by reactiveion etching, plasma etching, ion milling, or by any other directionalprocess. In addition, the etch-stop pit 516 may be formed by othermethods such as isotropic or anisotropic etching, so long as theetch-stop pit 516 attains the desired shape and depth.

Having created the etch-stop pit 516, the surfaces of the etch-stop pit516 are covered, preferably conformally, with an etch-stop layer 458, atstep 4030, as illustrated in FIG. 47. The etch-stop layer 458 may beconveniently provided by thermally oxidizing the substrate to provide anetch-stop layer 458 comprising silicon dioxide. An appropriate choicefor the etch-stop layer 458 includes any material that is resistant tothe etchant which will be used to create the V-groove 512. During thethermal oxidation step 4030, the previously deposited silicon dioxideprotective layer 452 increases in thickness and surrounds the perimeterof the aperture definition layer 454, as illustrated in FIG. 47.

With the etch-stop layer 458 in place, processing continues by removing,at step 4040, the aperture definition layer 454 to provide a V-grooveaperture 455 in the protective layer 452, as shown in FIG. 48. Asufficient thickness of the protective layer 452 is removed, at step4050, to expose the surface of the substrate 450 disposed below theaperture definition layer 454 so that the V-groove aperture 455communicates with the surface of the substrate 450. A portion of theprotective layer 452 and the etch-stop layer 458 remain on the surfaceswhere the V-groove 512 will not be formed, as illustrated in FIG. 49. Asuitable process for removing a thickness of the protective layer 452 isa short duration, wet or dry, oxide etch.

Next, as shown in FIG. 50, the portion substrate 450 accessible throughthe V-groove aperture 455 is selectively removed, at step 4060, to formthe V-groove 512, as illustrated in FIG. 50. Appropriate processes forthe formation of the V-groove 512 include anisotropic etching with EDPor TMAH. KOH may also be used; however, since KOH can attack theprotective layer 452 and etch-stop layer 458, KOH should only be used ifthe protective layer 452 and etch-stop layer 458 are sufficiently thickso as not to be completely removed by the KOH. As a final optional step,the remaining portions of the protective layer 452 and etch-stop layer458 may be removed at step 4070, to yield the device illustrated in FIG.51.

Referring now to FIGS. 41 and 52–58, another method in accordance withthe present invention is illustrated for creating the device shown inFIGS. 43 and 44. As illustrated in FIG. 52, a substrate 520 made from<100>-oriented Si is provided. The processing of the substrate 520begins at step 4100 of FIG. 41 by providing a first protective layer 522on a first surface of the substrate 520 to cover that portion of thesubstrate 520 in which neither the etch-stop pit 586 nor the V-groove582 is to be provided.

The first protective layer 522 may be deposited over the entire surfaceof the substrate 520. Thereafter, portions of the first protective layer522 may be removed to expose the surface of the substrate 520 at theselected areas for the etch-stop pit 586 and the V-groove 582. Thematerial of the first protective layer 522 is chosen to be resistant tothe etchant that will be used to form the V-groove 582. For example,silicon nitride is one suitable material. The silicon nitride may bedeposited by CVD.

Following the application of the first protective layer 522, a secondprotection layer 524 is deposited, at step 4110, over a selected portionof the first protective layer 522 and the substrate surface where theV-groove 582 is to be formed, as shown in FIG. 52. The second protectionlayer 524 includes an aperture 521 through which the etch-stop pit 586may be formed.

Processing continues with the selective removal, at step 4120, of aportion of the substrate 520 located within the etch-stop pit aperture521 to form an etch-stop pit 586 in the substrate 520, as depicted inFIG. 53. The etch-stop pit 586 may conveniently be formed by reactiveion etching, plasma etching, ion milling, or by any other directionalprocess. In addition, the etch-stop pit 586 may be formed by othermethods such as isotropic or anisotropic etching, so long as theetch-stop pit 586 attains the desired shape and depth.

Having created the etch-stop pit 586, the surfaces of the etch-stop pit586 and second protective layer 524 are covered, preferably conformally,with an etch-stop layer 528, at step 4130, as illustrated in FIG. 54.The etch-stop layer 528 may be conveniently provided by CVD. Anappropriate choice for the etch-stop layer 528 includes any materialthat is resistant to the etchant which will be used to create theV-groove 582, such as silicon nitride.

With the etch-stop layer 528 in place, processing continues by removing,at step 4140, the portion of the etch-stop layer 528 disposed on theupper surface 541 of second protective layer 524. The portion of theetch-stop layer 528 disposed within the etch-stop pit 586 is retained,as illustrated in FIG. 55. The removal step 4140 may be performed by anysuitable method such as planarization or polishing. Subsequently, atstep 4150, a second protective layer 524 is removed, as shown in FIG.56, to provide a V-groove aperture 525. A suitable method for removingthe second protective layer 524 includes etching with dilute HF.

Next, as shown in FIG. 57, the portion substrate 520 accessible throughthe V-groove aperture 525 is selectively removed, at step 4160, to formthe V-groove 582, as illustrated in FIG. 50. Appropriate processes forthe formation of the V-groove 582 include anisotropic etching with KOH.As a final optional step, the remaining portions of the first protectivelayer 522 and etch-stop layer 528 may be removed at step 4170, to yieldthe device illustrated in FIG. 58.

Referring now to FIGS. 42 and 59–64, yet another method in accordancewith the present invention is illustrated for creating the device shownin FIGS. 43 and 44. As illustrated in FIG. 59, a substrate 590 made from<100>-oriented Si is provided. The processing of the substrate 590begins at step 4200 of FIG. 42 by providing protective an aperturedefinition layer 594 deposited over a selected portion of the substrate590, as shown in FIG. 59. The location of the aperture definition layer594 is selected to cover that portion of the substrate surface at whichthe V-groove 632 is to be located. A suitable material for use as theaperture definition layer 524 is silicon nitride.

The processing of the substrate 590 continues, at step 4210, byproviding a photoresist layer 592 over the aperture definition layer 594and over the portions of the substrate 590 not covered by the aperturedefinition layer 524. Photoresist layer 592 is patterned, using methodsknown in the art, to provide an etch-stop pit aperture 591, asillustrated in FIG. 59. Processing continues with the selective removal,at step 4220, of a portion of the substrate 590 located within theetch-stop pit aperture 591 to form an etch-stop pit 636 in the substrate590, as depicted in FIG. 60. The etch-stop pit 636 may conveniently beformed by a process which does not remove the aperture definition layer594. In addition, the etch-stop pit 636 may be formed by other methodssuch as isotropic or anisotropic etching, so long as the etch-stop pit636 attains the desired shape and depth.

Having created the etch-stop pit 636, the photoresist layer 592 isremoved, at step 4230. The surfaces of the etch-stop pit 636 and exposedsurfaces of the substrate 590 are oxidized to form an etch-stop layer598, at step 4230, as illustrated in FIG. 61. With the etch-stop layer598 in place, processing continues by removing, at step 4240, theaperture definition layer 594 to provide an un-oxidized region 597 ofthe substrate 590, as shown in FIG. 62.

Next, as shown in FIG. 63, the un-oxidized region 597 of the substrate590 is selectively removed, at step 4250, to form the V-groove 632, asillustrated in FIG. 63. Appropriate processes for the formation of theV-groove 632 include anisotropic etching with EDP or TMAH. KOH may alsobe used; however, since KOH can attack oxide etch-stop layer 598, KOHshould only be used if the etch-stop layer 598 is sufficiently thick soas not to be completely removed by the KOH. As a final optional step,the remaining portions of the etch-stop layer 598 may be removed at step4260, to yield the device illustrated in FIG. 64.

These and other advantages of the present invention will be apparent tothose skilled in the art from the foregoing specification. Accordingly,it will be recognized by those skilled in the art that changes ormodifications may be made to the above-described embodiments withoutdeparting from the broad inventive concepts of the invention. Forexample, a non-anisotropically etched feature may be formed adjacent anetch-stop pit. It should therefore be understood that this invention isnot limited to the particular embodiments described herein, but isintended to include all changes and modifications that are within thescope and spirit of the invention as set forth in the claims.

1. A method for micromachining <100> silicon, comprising: (a) definingthree areas of a substrate: an unetched area, a dry etch area, and a wetetch area, wherein the dry etch area and the wet etch area are adjacent;(b) forming an SiO₂ layer over the unetched and wet etch areas of asubstrate, forming silicon nitride on the SiO₂ in the wet etch area,wherein the dry etch area is uncovered; (c) dry etching the dry etcharea to form a dry-etched pit; (d) oxidizing the substrate to form aSiO₂ layer in the dry-etched pit; (e) removing the silicon nitride andthinning the SiO₂ to expose the wet etch area; and (f) wet etching thewet etch area.
 2. The method of claim 1, further comprising removingSiO₂ after (f).
 3. A method for micromachining <100> silicon,comprising: (a) defining three areas of a substrate: an unetched area, adry etch area, and a wet etch area, wherein the dry etch area and thewet etch area are adjacent; (b) forming a silicon nitride/SiO₂ layerover the unetched area, with the SiO₂ on top, forming an SiO₂ layer overthe wet etch area, wherein the dry etch area is uncovered; (c) dryetching the dry etch area to form a dry-etched pit; (d) conformallycoating the substrate with a hard mask material to form a hard masklayer in the dry-etched pit; (e) removing the SiO₂ from the substrate sothat the wet etch area is exposed; and (f) wet etching the wet etcharea.
 4. The method of claim 3, further comprising removing hard maskmaterial after (f).
 5. The method of claim 3, wherein the hard maskmaterial is chemical vapor deposited nitride.
 6. A method formicromachining <100> silicon, comprising: (a) defining three areas of asubstrate: an unetched area, a dry etch area, and a wet etch area,wherein the dry etch area and the wet etch area are adjacent; (b)forming a photoresist layer over the unetched area, forming a hard masklayer over the wet etch area, wherein the dry etch area is uncovered;(c) dry etching the dry etch area to form a dry-etched pit; (d) removingthe photoresist; (e) oxidizing the substrate to form a SiO₂ layer in thedry-etched pit; (f) removing the hard mask to expose the wet etch area;and (g) wet etching the wet etch area.
 7. The method of claim 6, whereinthe hard mask material comprises silicon nitride.
 8. The method of claim6, further comprising removing SiO₂ after (g).